EVALUATION OF THE PARAMETERS OF SIMPLE ELASTOPLASTIC CONSTITUTIVE MODELS FOR DHAKA CLAY

Abstract

Geotechnical engineering problems are usually solved by considering soil as an elastic or rigid plastic material where stress-dilatancy characteristics are not taken into considerations. As actual soil is an elastoplastic material most of the designs are either over designed or under designed for not taking proper considerations of constitutive model, i.e., stress – strain relation. On the contrary, most of the constitutive models are not able to simulate total behavior of the soil or very complex in nature where parameter determination requires sophisticated tests. Dhaka city and its adjacent Gazipur and Savar is placed on the southern part of Madhupur tract the oldest sediment deposit. As a rapid expanding city, it often require to solve complex geotechnical problems where geotechnical engineers have to deal with the Dhaka soil. In this research work two well-known constitutive models, i.e., Subloading tij model and Cam-clay model were considered to apply them on Dhaka clay and observe their performance on capturing the soil behavior. This study focuses on collecting, testing and modelling of soil at undisturbed condition to get the best usability and application. Total six undisturbed soil samples were collected from various depth of Dhaka, Savar and Gazipur. Consolidated drained and undrained tests were performed under constant confining pressure to observe the stress-strain characteristics of Dhaka clay at various OCR. To evaluate the model parameters one dimensional consolidation tests were conducted for each locations. From the test results of one dimensional consolidation test, the compression index were found in the range of 0.057 to 0.07 and swelling index is found in the range of 0.01 to 0.015. From triaxial compression test the principal critical stress ratio were found in the range of 2.5 to 3.3 for various locations. From the simulations the density parameter were found in the range of 300 to 480 and bonding parameters in the range of 10 to 15 for drained condition and 30 to 45 for undrained condition respectively. To observe the performance of the models considered, the simulated results using the above parameters were plotted with the test results. It was found that for all the stress conditions and stress paths, Subloading tij model can capture the Dhaka clay behavior accurately at drained and undrained conditions. On the other hand Cam-clay and iv Modified Cam-clay model have failed to capture the Dhaka clay behavior at undained condition. At drained condition Modified Cam-clay model can capture the soil strength behavior satisfactorily at normally consolidated condition. At all stress conditions, the volumetric strain simulated by both Cam-clay and Modified Cam-clay model were found to overestimate the observed test results.